Longevity (as a measure of resistance to irradiation) has been examined after different doses of 60Co γ-rays, in four inbred lines and their hybrids in D. melanogaster. The control data showed some additive and non-additive effects. At 40, 60, 80, 100 and 120 krads, non-additive effects were highly significant, but additive effects were significant only at 100 and 120 krads, in particular the latter. Thus the genetic architecture varies with dose. Detailed genetic studies probably are simplest to carry out at high doses, because of the importance of additive effects.